The present invention relates to laser systems and in particular to a method of and apparatus for combining plural laser sources.
The investigation of, e.g., biological processes and functions using laser-based fluorescence instruments is becoming increasingly common. Such instruments measure and observe the fluorescence signals emitted when a target is, e.g., illuminated or excited by a laser source. Typical applications of these techniques include, but are not limited to, fluorescence imaging, scanning microscopy, confocal microscopy, total internal reflection fluorescence microscopy (TIRF), fluorescence correlation spectroscopy (FCS), flow cytometry, imaging cytometry, small animal or molecular imaging, high content screening and cellular imaging.
Laser-based fluorescence instruments typically require combinations of laser light of different wavelengths, e.g., in order to excite different fluorescent targets such as fluorescent probes and fluorescent proteins. The experiments and investigations carried out using these instruments can involve single or multiple combinations of fluorescent targets.
The increasing diversity and sophistication of applications for these instruments has led to an increasing demand for laser sources featuring as wide a range of wavelengths as possible, together with enhanced flexibility, control, repeatability, reliability and precision of the combined laser source.
To meet these demands, it is usually desirable that the laser sources used for excitation can be selected and controlled with a high degree of precision, and that, e.g., the desired intensity levels requested by a user can be maintained within very tight tolerances both for the duration of any given experiment, and over the longer term use of the instrument. These requirements can also extend to other key parameters of the laser system, such as pointing stability, polarisation stability and noise performance.
A number of laser systems have previously been proposed to try to meet these requirements.
One known type of such laser systems can be viewed as “free space” or “direct-coupled” laser systems. In these systems, light from plural laser sources is co-aligned into a “single” beam using combining mirrors, with the combined beam then being coupled directly (i.e. in free space) into the main instrument.
A difficulty with such direct-coupled systems is that it can be difficult to ensure precise co-alignment of the laser beams in free space, and to maintain that alignment over time.
It is known therefore to also use an optical fibre to couple the combined beam from the laser sources into the main instrument. In these arrangements, the laser beams are first combined in free space using, e.g., mirrors, but that beam is then coupled into an optical fibre which delivers it to the main target instrument. In this way, the free space travel of the combined laser beams (and hence the effect of any laser pointing or alignment errors) can be minimised. Such systems are commonly referred to as being “fibre-coupled”.
In both direct-coupled and fibre-coupled systems it is necessary to provide some mechanism whereby, e.g., individual or selected laser lines from the plural laser sources can be selected for use in the instrument, and, e.g., the intensity of the beam input to the target instrument can be controlled.
One known way to provide such selection and control is to place appropriate filters in the path of the combined beam (or, indeed, in the paths of the individual beams from the individual laser sources). These filters could comprise, e.g., dielectric filters that can be placed in the beam path, or more sophisticated devices such as acoustic-optical tuneable filters (AOTFs).
However, the use of such filters does inevitably lead to transmission losses at the filter (which losses can be of the order of 20%). The laser switching speed of filter-based systems can also be relatively limited, as can their laser line blocking (filtering) performance, and the wavelength range over which they can be used.
Many laser-based fluorescence instruments use diode lasers. When the output intensity of a diode laser is reduced towards zero, a threshold point is reached where the diode laser switches from a lasing mode with predominantly stimulated emission, to a “light emitting diode” (LED) mode with incoherent light output generated by spontaneous emission. Just below this lasing threshold point, all the optical output power is emitted in the LED mode, and just above the lasing threshold point the desired laser mode starts to dominate and the LED mode emission narrows and reduces in level.
Any LED emission by a diode laser is undesirable in a fluorescence instrument, since it will interfere with the fluorescence measurements. For this reason, diode lasers are usually operated well above their threshold points in laser combining systems, so as to avoid any risk of LED emission. Filters are then used, as discussed above, to reduce and/or select the desired laser output.